PHP:如何确保输入到表单中的句子至少有20个字符，最后9个字符被拉到新的页面上显示？

<?php
if(isset($_POST['submit'])){
    $sentence = $_POST['sentence'];
    if(strlen($sentence) < 20){
        $last_chars = substr($sentence, -9);
        header("Location: new_page.php?last_chars=$last_chars");
        exit();
    }
}
?>

<form method="POST" action="">
    <label for="sentence">请输入句子：</label>
    <input type="text" name="sentence" id="sentence" required>
    <input type="submit" name="submit" value="提交">
</form>

<?php
if(isset($_GET['last_chars'])){
    $last_chars = $_GET['last_chars'];
    echo "最后9个字符是：$last_chars";
}
?>